The invention relates to a colour display tube comprising in an evacuated envelope an electron gun system of the "in-line" type for generating three electron beams situated with their axes in one plane, the axis of the central beam coinciding with the tube axis. The electron beams converge on a display screen provided on a wall of the envelope and in the operating display tube are deflected over the display screen in two mutually perpendicular directions by means of a first and a second deflection field. The direction of the first deflection field is parallel to the plane and the electron gun system comprises at its end field shapers for causing the rasters written on the display screen by the electron beams to coincide as much as possible.
Such a colour display tube is disclosed in U.S. Pat. No. 4,196,370. A frequently occurring problem in colour display tubes having an electron gun system of the "in-line" type is coma, meaning that the dimensions of the rasters which are written on the display screen by the three electron beams are different. This is the result of the eccentric location of the outermost electron beams relative to the vertical deflection field (the frame deflection field). In the patent specification a large number of patents are mentioned in which partial solutions are given. These solutions consist of using magnetic field conducting and/or screening rings and plates which are mounted at the end of the gun and which intensify or weaken the deflection field or the deflection fields locally along a part of the paths of the electron beams. With a number of these means it is possible to cause the rasters written on the display screen by the three electron beams to coincide substantially. A disadvantage of the use of such means, however, is that defocusing occurs in the outermost beams during deflection which is expressed in a distorted spot on the display screen which is surrounded by a haze. One of the patents is U.S. Pat. No. 3,594,600 which describes a colour display tube in which the rasters written by the three electron beams are made to coincide by placing two elongate C-shaped magnetic screens beyond the outermost electron beams. As a result of this the outermost electron beams are screened from the edge field of the line deflection field (the vertical field lines) while the edge field is admitted to the central electron beam. The three electron beams are screened from the edge field of the frame deflection field (the horizontal field line) which is guided entirely around the three beams. Thus these field shapers influence the line coma and not the field coma.
In Netherlands Patent Application No. 7801317, corresponding to U.S. Pat. No. 4,237,437, a system of deflection coils is described in which field-forming means are provided in the system of deflection coils. They consist, for example, of two soft magnetic elements which are provided diametrically opposite to each other beyond the line deflection coil, substantially transversely to the magnetic field of the frame deflection coil on the side of the neck of the system of deflection coils. A disadvantage of the use of such field-forming means is that a large part of the frame deflection field is distorted by the field-forming elements, which consume substantial deflection energy.